feat: GFPGAN face-identity restoration post-pass

Add an optional, commercial-safe face-restoration post-pass that recovers
face identity the diffusion removal pass drifts (canny holds structure, not
likeness) while still scrubbing the pixel watermark in the face regions.

- face_restore.py: GFPGANer singleton (CPU unless CUDA), the basicsr
  torchvision.transforms.functional_tensor shim, and the pure feather
  _composite_faces helper (unit-tested without the model). GFPGAN
  re-synthesizes each face from a StyleGAN2 prior, so composited face pixels
  are GAN-generated (no watermark, no pixel-copy) -- oracle-clean at weight 0.5
  with identity preserved.
- InvisibleEngine.remove_watermark: restore_faces / restore_faces_weight,
  best-effort, auto-skips when the extra is absent or no face is detected.
- CLI --restore-faces/--no-restore-faces + --restore-faces-weight on
  invisible/all/batch (on by default).
- restore extra (gfpgan/facexlib/basicsr), numpy<2-pinned (scipy<1.18,
  numba<0.60) and kept out of `all`; basicsr needs Python <3.13 + setuptools<69
  to build, so pin .python-version 3.12.

Commercial-safe: GFPGAN Apache-2.0, RetinaFace MIT. The CodeFormer alternative
is non-commercial and is not shipped. The earlier IP-Adapter FaceID layer was
removed (footgun: needs high strength, corrupts faces at the low removal
strength).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.python-version

@@ -0,0 +1 @@
+3.12